TIR-based optical touch systems of projection-type

ABSTRACT

A touch-sensitive apparatus operates by light frustration (FTIR) and comprises a light transmissive panel that defines a front surface and a rear surface, light emitters optically connected to the panel so as to generate light that propagates by total internal reflection inside the panel, and light detectors optically connected to the panel so as to define a grid of propagation paths inside the panel between pairs of light emitters and light detectors. Each of said light emitters is a VCSEL array, each said VCSEL array including a plurality of VCSELs driven in parallel to collectively form one light emitter. Preferably, each light detector is optically connected to the panel via an angular filter, tailored to pass light to the detector in an angular range in which the emitters operate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit and priority to and is a U.S.National Phase of PCT International Application NumberPCT/SE2015/050044, filed on Jan. 16, 2015. This application claims thebenefit and priority to Swedish patent application No. 1450039- 1, filed16 Jan. 2014. The disclosure of the above-referenced applications arehereby expressly incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention generally relates to optical touch-sensingsystems, and in particular to such systems that operate by projectionmeasurements of light that propagates by total internal reflection (TIR)inside a light transmissive panel.

BACKGROUND ART

Touch-sensing systems (“touch systems”) are in widespread use in avariety of applications. Typically, the touch systems are actuated by atouching object such as a finger or stylus, either in direct contact, orthrough proximity (i.e. without contact), with a touch surface. Touchsystems are for example used as touch pads of laptop computers, incontrol panels, and as overlays to displays on e.g. hand held devices,such as mobile telephones. A touch panel that is overlaid on orintegrated in a display is also denoted a “touch screen”. Many otherapplications are known in the art.

There are numerous known techniques for providing touch sensitivity,e.g. by incorporating resistive wire grids, capacitive sensors, straingauges, etc into a touch panel. There are also various types of opticaltouch systems, which e.g. detect shadows cast by touching objects onto atouch surface, or detect light scattered off the point(s) of touchingobjects on a touch panel.

One specific type of optical touch system uses projection measurementsof light that propagates on a plurality of propagation paths inside alight transmissive panel that defines a touch surface. The projectionmeasurements thus quantify a property, e.g. power, of the light on theindividual propagation paths, when the light has passed the panel. Thelight propagates inside the panel by total internal reflection (TIR)against the touch surface, such that objects on the touch surface causesthe propagating light on one or more propagation paths to be attenuated,commonly denoted FTIR (Frustrated Total Internal Reflection). For touchdetermination, the projection measurements may be processed by simpletriangulation, or by more advanced image reconstruction techniques thatgenerate a two-dimensional distribution of disturbances on the touchsurface, i.e. an “image” of everything on the touch surface that affectsthe measured property. Examples of such touch systems are found in U.S.Pat. Nos. 3,673,327, 4,254,333, 6,972,753, 7,432,893, US2006/0114237,US2007/0075648, WO2009/048365, US2009/0153519, WO2010/006882,WO2010/064983, WO2010/134865 and WO2012/105893.

The prior art suggests several different approaches for introducing thelight into the panel and for detecting the light downstream of the touchsurface. For example, U.S. Pat. No. 7,432,893 proposes coupling lightinto the panel via revolved prisms that are attached to the rear surfaceof the panel, and detecting the light at photodetectors that aredirectly attached to the front surface of panel. In WO2010/064983, lightis coupled into and out of the panel via the edge surface that connectsthe front and rear surfaces of the panel, or via wedges that areattached to the front or rear surface of the panel. In WO2012/105893, asheet-like microstructured element, e.g. a tape of light transmissivematerial, is provided on the front or rear surface of the panel forcoupling light into and out of the panel.

One challenge when designing an optical touch system of this type is toenable consistent touch determination despite the fact that thedetectors need to detect small changes in weak optical signals inpresence of potentially significant interferences that affect thereliability of the optical signals. One such interference is caused byambient light, e.g. from sunlight or residential lighting, that mayimpinge on the detectors and influence the optical signals. Anotherinterference is caused by accumulation of contamination on the touchsurface, such a fingerprints, drops of saliva, sweat, smear, liquidspills, etc. The contamination will interact with the propagating lightand cause changes to the optical signals that may be difficult todistinguish from changes caused by “true objects”, e.g. objects that areactively manipulated in contact with the touch surface.

In touch-sensitive devices, there is also a general trend to avoidattaching components to the front surface. These components may form aframe around the touch-sensitive region and thereby reduce the ratio ofthe active area (the surface area that is available for touchinteraction) to the total area of the touch-sensitive device.Furthermore, if the components protrude from the front surface of thepanel, it may be necessary to provide a bezel at the perimeter of thepanel to protect and hide the components and possibly any wiringconnected to the components. Given the nature of user interaction withtouch-sensitive devices, such a bezel may disrupt the user experienceand even prevent certain types of interaction. The bezel may also causedirt and other contaminants to accumulate in the area where the bezeljoins the panel. To overcome this problem, it is desirable to designtouch systems for flush mount of the panel in the supporting frame ofthe touch-sensitive device, i.e. such that the front surface of thepanel is essentially level with the surrounding frame material. This isalso known as “edge-to-edge”.

In aforesaid U.S. Pat. No. 7,432,893, the impact of ambient light isreduced by attaching the photodetectors to the front surface, such thatthe photodetectors face away from the ambient light that enters thepanel through the front surface. This solution requires a significantbezel to hide and protect the photodetectors and the associated wiring.U.S. Pat. No. 7,432,893 also proposes to intermittently measure ambientlevels at the photodetectors and compensate the respective projectionmeasurement for the measured ambient level.

The influence of contamination may be handled by dedicated signalprocessing that actively estimates the influence of contamination overtime and compensates for this influence, e.g. as disclosed inWO2011/028169, WO2011/049512 and WO2012/121652.

However, in view of the weak optical signals and small attenuationcaused by touching objects, there is room for further improvement whenit comes to increasing the robustness of the touch system to ambientlight and contamination on the touch surface.

SUMMARY

It is an objective of the invention to at least partly overcome one ormore of the above-identified limitations of the prior art.

Another objective is to provide a touch-sensitive apparatus that has areduced sensitivity to ambient light.

A further objective is to provide a touch-sensitive apparatus that has areduced sensitivity to contamination on the touch surface.

One or more of these objectives, as well as further objectives that mayappear from the description below, are at least partly achieved by atouch-sensitive apparatus according to the independent claims,embodiments thereof being defined by the dependent claims.

A first aspect of the invention relates to a touch-sensitive apparatus,comprising: a light transmissive panel that defines a front surface andan opposite, rear surface; a plurality of light emitters opticallyconnected to the panel so as to generate light that propagates by totalinternal reflection inside the panel across a touch-sensitive region onthe panel; a plurality of light detectors optically connected to thepanel so as to define a grid of propagation paths across thetouch-sensitive region between pairs of light emitters and lightdetectors; wherein each of said light emitters is a VCSEL array, eachsaid VCSEL array including a plurality of VCSELs driven in parallel tocollectively form one light emitter.

In one embodiment each light detector is optically connected to thelight transmissive panel via an angular filter which is applied to anoutcoupling region of the light transmissive panel and is configured totransmit the propagating light only within a confined range of angleswith respect to the normal of the outcoupling region; and wherein eachlight emitter (2) is optically connected to an incoupling region of thelight transmissive panel and is configured to transmit a beam of lightfor total internal reflection so as to reach the outcoupling region atleast partially within said confined range of angles.

In one embodiment a bandpass filter, tailored to an operating wavelengthfor said VCSELs, is arranged at said detectors.

In one embodiment said bandpass filter has a bandwidth of less than 5nm.

In one embodiment the confined range extends from a lower angle limitθ_(min) to an upper angle limit θ_(max), wherein the lower angle limitθ_(min) is equal to or larger than a critical angle θ_(c), which isgiven by θ_(c)=arcsin(1/n_(panel)), with n_(panel) being the refractiveindex of the light transmissive panel at the outcoupling region.

In one embodiment said outcoupling region is arranged on at least one ofthe front and rear surfaces.

In one embodiment said incoupling region is arranged on at least one ofthe front and rear surfaces.

In one embodiment said incoupling region is arranged on a side edge ofthe panel, connecting the front and rear surfaces.

In one embodiment the lower angle limit θ_(min) exceeds the criticalangle by an angle Δθ, which is at least 5°, 10° or 15°.

In one embodiment the lower angle limit θ_(min) is equal to or largerthan a first cut-off angle θ_(w)=arcsin(n_(w)/n_(panel)) with n beingthe refractive index of water, n_(panel)>n_(w).

In one embodiment the lower angle limit θ_(min) is equal to or largerthan a second cut-off angle θ_(f)=arcsin(n_(f)/n_(panel)), with n_(f)being the refractive index of finger fat, n_(panel)>n_(f).

In one embodiment the angular filter (20) is configured as a dielectricmultilayer structure.

In one embodiment the light transmissive panel (1) is mounted onto afront surface of a display device (34) by a lamination layer (36) oflight transmissive material which is arranged in contact with the rearsurface (6) of the light transmissive panel (1) and the front surface ofthe display device (34), and wherein the lower angle limit θ_(min) isequal to or larger than a laminate cut-off angleθ_(c,l)=arcsin(n_(lam)/n_(panel)), with n_(lam) being the refractiveindex of lamination layer (36), n_(lam)<n_(panel).

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described in more detail withreference to the accompanying schematic drawings.

FIG. 1 is a section view of a light transmissive panel to illustrate theprinciple of using TIR for touch detection.

FIG. 2A is a top plan view of a touch-sensitive apparatus according toan embodiment.

FIG. 2B is a 3D plot of an attenuation pattern generated based on energysignals from a TIR-based projection-type touch-sensitive apparatus.

FIG. 3A is section view of a VCSEL, illustrating its internal functionalstructure.

FIG. 3B is an elevated view of an emitter in the form of a VCSEL array.

FIG. 4 is a section view of a light outcoupling structure according to afirst embodiment.

FIG. 5A is a plot of transmission as a function of incidence angle foran angular filter included in the first embodiment, and FIG. 5B is aperspective view of the range of angles that are transmitted by theangular filter.

FIG. 6 is a section view to illustrate characteristic angles of theangular filter and characteristic angles of the panel.

FIG. 7 is a section view to illustrate incoupling of ambient light viacontamination on the touch surface.

FIG. 8 is a section view to illustrate a relation between bounce anglesin a two-layer panel.

FIG. 9 is a section view to illustrate bounce angles in a panel which islaminated to a display device.

FIG. 10A is a section view of a light outcoupling structure according toa second embodiment.

FIGS. 10B-10C are section and plan views of a light outcouplingstructure according to a third embodiment.

FIGS. 10D-10H are section views of variations of the first to thirdembodiments.

FIGS. 11A-11B are section and plan views of a first alternativeoutcoupling structure.

FIGS. 12A-12B are section and plan views of a second alternativeoutcoupling structure.

FIGS. 13A-13B are section and plan views of a light incoupling structureaccording to a first embodiment, and FIG. 13C is a section view of avariation of the first embodiment.

FIG. 14 is a bottom plan view of a sequence of structures arranged on apanel to couple light into and/or out of the panel.

FIG. 15 is a graph of light reflectivity at a panel-water interface as afunction of angle of incidence inside the panel.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following, various inventive light coupling structures will bepresented as installed in an exemplifying TIR-based projection-typetouch-sensitive apparatus. Throughout the description, the samereference numerals are used to identify corresponding elements.

FIG. 1 illustrates the concept of touch detection based on attenuationof propagating light, commonly denoted FTIR (Frustrated Total InternalReflection). According to this concept, light is transmitted inside apanel 1 along a plurality of well-defined propagation paths. The panel 1is made of solid material in one or more layers and may have any shape.The panel 1 defines an internal radiation propagation channel, in whichlight propagates by internal reflections, also denoted “bounces” in thefollowing. In the example of FIG. 1, the propagation channel is definedbetween the boundary surfaces 5, 6 of the panel 1, and the front surface5 allows the propagating light to interact with touching objects 7 andthereby defines the touch surface 4. The interaction is enabled byinjecting the light into the panel 1 such that the light is reflected bytotal internal reflection (TIR) in the front surface 5 as it propagatesthrough the panel 1. The light may be reflected by TIR in the rearsurface 6 or against a reflective coating thereon. It is alsoconceivable that the propagation channel is spaced from the rear surface6, e.g. if the panel comprises multiple layers of different materials.The panel 1 may thus be made of any solid material (or combination ofmaterials) that transmits a sufficient amount of light in the relevantwavelength range to permit a sensible measurement of transmitted energy.Such material includes glass, poly(methyl methacrylate) (PMMA) andpolycarbonates (PC). The panel 1 typically has a refractive index in therange of 1.3-1.7. For example, PMMA has a refractive index of about 1.5and PC has a refractive index of about 1.6 in the near infrared (NIR).The panel 1 may be designed to be overlaid on or integrated into adisplay device or monitor (not shown).

As shown in FIG. 1, an object 7 that is brought into close vicinity of,or in contact with, the touch surface 4 may interact with thepropagating light at the point of touch. In this interaction, part ofthe light may be scattered by the object 7, part of the light may beabsorbed by the object 7, and part of the light may continue topropagate in its original direction across the panel 1. Thus, thetouching object 7 causes a local attenuation or “frustration” of thetotal internal reflection, which leads to a decrease in the energy (orequivalently, the power or intensity) of the transmitted light, asindicated by the thinned lines downstream of the touching objects 7 inFIG. 1.

FIG. 2A illustrates an example embodiment of a touch-sensitive apparatus100 that is based on the concept of FTIR. Emitters 2 are distributedalong the perimeter of the touch surface 4, beneath the panel 1, toproject light onto an incoupling structure on the panel 1 such that atleast part of the light is captured inside the panel 1 for propagationby internal reflections in the propagation channel. Detectors 3 aredistributed along the perimeter of the touch surface 4, beneath thepanel 1, and are optically coupled to the panel 1 so as to receive partof the propagating light from a respective outcoupling structure. Thelight from each emitter 2 will thereby propagate inside the panel 1 to anumber of different detectors 3 on a plurality of light propagationpaths D. Even if the light propagation paths D correspond to light thatpropagates by internal reflections inside the panel 1, the lightpropagation paths D may conceptually be represented as “detection lines”that extend across the touch surface 4 between pairs of emitters 2 anddetectors 3, as indicated by dashed lines in FIG. 2A. Thus, thedetection lines correspond to a projection of the propagation paths ontothe touch surface 4. Thereby, the emitters 2 and detectors 3collectively define a grid of detection lines (“detection grid”) on thetouch surface 4, as seen in a top plan view. It is appreciated that FIG.2A is an example, and that a (significantly) larger number of emitters 2and/or detectors 3 may be included in the apparatus 100. Also, thedistribution of emitters 2 and detectors 3 may differ.

The detectors 3 collectively provide an output signal, which is receivedand samp-led by a signal processor 10. The output signal contains anumber of sub-signals, also denoted “projection signals”, eachrepresenting the energy of light received by a certain light detector 3from a certain light emitter 2. Depending on implementation, the signalprocessor 10 may need to process the output signal for separation of theindividual projection signals. The projection signals represent theenergy, intensity or power of light received by the detectors 3 on theindividual detection lines D. Whenever an object touches a detectionline, the received energy on this detection line is decreased or“attenuated”.

The signal processor 10 may be configured to process the projectionsignals so as to determine a property of the touching objects, such as aposition (e.g. in the x,y coor-dinate system shown in FIG. 2A), a shape,or an area. This determination may involve a straight-forwardtriangulation based on the attenuated detection lines, e.g. as disclosedin U.S. Pat. No. 7,432,893 and WO2010/015408, or a more advancedprocessing to recreate a distribution of attenuation values (forsimplicity, referred to as an “attenuation pattern”) across the touchsurface 1, where each attenuation value represents a local degree oflight attenuation. An example of such an attenuation pattern is given inthe 3D plot of FIG. 2B, where the peaks of increased attenuationrepresent touching objects. The attenuation pattern may be furtherprocessed by the signal processor 10 or by a separate device (not shown)for determination of a position, shape or area of touching objects. Theattenuation pattern may be generated e.g. by any available algorithm forimage reconstruction based on projection signal values, includingtomographic reconstruction methods such as Filtered Back Projection,FFT-based algorithms, ART (Algebraic Reconstruction Technique), SART(Simultaneous Algebraic Reconstruction Technique), etc. Alternatively,the attenuation pattern may be generated by adapting one or more basisfunctions and/or by statistical methods such as Bayesian inversion.Examples of such reconstruction functions designed for use in touchdetermination are found in WO2009/077962, WO2011/049511, WO2011/139213,WO2012/050510, and WO2013/062471, all of which are incorporated hereinby reference.

In the illustrated example, the apparatus 100 also includes a controller12 which is connected to selectively control the activation of theemitters 2 and, possibly, the readout of data from the detectors 3.Depending on implementation, the emitters 2 and/or detectors 3 may beactivated in sequence or concurrently, e.g. as disclosed inWO2010/064983. The signal processor 10 and the controller 12 may beconfigured as separate units, or they may be incorporated in a singleunit. One or both of the signal processor 10 and the controller 12 maybe at least partially implemented by software executed by a processingunit 14, such as a CPU.

Embodiments of emitters and structures for incoupling of light will nowbe explained in detail with reference to FIGS. 3 and 13. Generally,these embodiments are presented in the context of the touch-sensitiveapparatus shown in FIG. 2A.

One of the main challenges in the art of optical touch sensing is theneed to obtain a usable optical signal at the detector side that issufficiently strong or characteristic so as to be recognizable frombackground noise. The emitters 2 may generate light in any wavelengthrange, but FTIR systems preferably operate in the near infrared (NIR),i.e. at wavelengths of about 750 nm-1400 nm, which is also the range inwhich the following emitter 2 examples generates light. As mentioned,both ambient light and changes to propagating light caused bycontamination on the touch surface are factors that must be taken intoaccount. Rather than solely trying to shield the detector side of thetouch-sensing system, the Applicant behind the present inventionsuggests that these problems can be alleviated by means of carefularrangement of the emitters 2. While other types of light sources havebeen suggested, emitters 2 are generally provided by means of lasers,typically edge-emitting diode lasers, in state of the art FTIR systems.Such diode lasers have several benefits, in particular with regard tocost and availability. However, it has been noted that even with highpower laser diodes, there is room for improvement in the signal to noisecharacteristics.

The present invention suggests providing emitters 2 in the form of VCSELarrays. A VCSEL, Vertical Cavity Surface-Emitting Laser, is asemiconductor laser which is well known in the art of light sources assuch, but which is significantly different from standard edge-emittingdiode lasers. FIG. 3A illustrates a typical VCSEL 21, with reference towhich the basic structure of this type of laser will be brieflydescribed. It should be noted that VCSELs can be built differently, thedesign of FIG. 3A being merely one example for the purpose of explainingthe general function thereof. The layered structure is built up from asubstrate 31, typically a GaAs wafer, and the structure comprises abottom mirror 33A, a top mirror 33B, with a laser cavity 35 in between.The mirrors 33A and 33B are created as a DBRs, Distributed BraggReflectors. The lower mirror 33A is typically an n-type DBR, and hasvery high reflectivity for the laser wavelength, typically 99.9%,whereas the top mirror 33B typically is a p-type DBR with about 99%reflectivity. This means that the illustrated embodiment is designed tobe a top emitting VCSEL 21. Alternatively, it is possible to construct abottom emitting VCSEL, which provides light out through the substrate31. The laser cavity 35 comprises at least one quantum well structure,i.e. a single quantum well, or several repeated structures forming amultiple quantum well. A typical example of a single quantum wellcomprises two AlGaAs layers, with a thin GaAs layer in between in whichquantum confinement is achieved. This is indicated in the drawing by agrey central layer of the laser cavity 35. Also, oxide layers 37 areformed between the laser cavity 35 and the respective DBRs 33A and 33B.Each oxide layer 37 is typically shaped with a circular aperture, whichprovides circular shaping of the laser beam that escapes through the topmirror 33B. The laser is electrically fed by means of cooperatingelectrodes; e.g. a bottom electrode 39A underneath the substrate 31, anda top electrode 39B for current injection. The top electrode 39B has anaperture for allowing the laser light to escape, and is provided bymeans for current injection, such as by the illustrated bonding wire39C.

VCSELs have certain benefits compared to traditional edge-emittinglasers, such as low threshold current, circular beam shape with lowdivergence, and good temperature stability. Still, these characteristicshave hitherto not made VCSELs preferable over edge-emitting laser diodesfor FTIR emitter purposes. However, VCSELS have another benefit overedge-emitting lasers that can be utilized, they are possible to buildinto 2D (two-dimensional) arrays. This is possible since they are grownon the surface of a wafer, whereas a side-emitting laser is cut out fromthe wafer, making at best a one-dimensional array possible. Also, theApplicant has found that a VCSEL array provides an advantage as anemitter 2 for FTIR purposes, other than simply including several VCSELsin one package.

FIG. 3B schematically shows an emitter 2 comprising a 2D VCSEL array,which in turn includes 9 individual VCSELs 21 in a 3×3 arrangement. EachVCSEL 21 may e.g. be of the type described with reference to FIG. 3A.However, the specific details of the VCSELs 21 are not illustrated inFIG. 3B for the sake of simplicity. All VCSELs 21 in the array aredriven in parallel, i.e. the respective top electrode 39B areinterconnected. In one embodiment, indicated by the dashed overheadlayer, a common conductive sheet 39D, such as a metal layer, may beprovided over the array structure with apertures for each VCSEL 21,either so as to operate as a common top electrode, or connecting to therespective top electrodes 39B of the individual VCSELs 21. The 3×3 arrayillustrated in FIG. 3B is merely an example to indicate the 2Ddisplacement of the individual VCSELs 21. In other embodiments, theemitter 2 may be formed of a 2D VCSEL array comprising hundreds ofindividual VCSELs 21. The pitch between two individual VCSELs 21 may beas small as in the order of single μm or larger. It should be noted thatthe individual VCSELs of the array need not be dispersed in a Cartesiantype grid as in FIG. 3B. In other embodiments, the individual VCSELs maybe arranged in a hexagonal grid, i.e. in parallel lines, each one offsethalf a pitch with respect to its neighboring lines. In yet anotherembodiment the individual VCSELs 21 may be arranged along concentriccircles, or in other shapes. Regardless of the specific grid character,it may be realized that in the near field immediately over the emitter 2the grid-like structure of the individual VCSELs 21 will characterizethe irradiance over the field angle of the emitter 2. However, the lightbeams 23 of the individual VCSELs 21 will blend within a very shortdistance to form an aggregate light beam of the emitter 2. A benefit ofthe VCSEL array is a consequence of this aggregation, which with properdesign may result in the unusual combination of a short coherence lengthand a narrow spectral bandwidth. The short coherence length results inreduced speckle noise at the detectors. Speckle patterns over the touchdetection surface and in the detector plane otherwise tend to lead tospeckle-induced noise. This is one of the reasons why the Applicant hasfound that a VCSEL array emitter 2 provides improved signal readout fortouch detection in FTIR systems over systems with a single laser emitterwith corresponding power.

Also, a VCSEL array emitter 2 typically provides a light beam with asubstantially circular cross-section with a narrow divergence of lessthan 20° (1/e²), even down to 15°. Preferred embodiments further buildon this fact, so as to improve signal detection at the detector 3 side,namely by restricting signal detection to a certain angular range withrespect to an outcoupling area of the panel where the detector 3 isconnected. Embodiments of structures for outcoupling and detection oflight will now be explained in detail with reference to FIGS. 4-12.Generally, these embodiments are presented in the context of thetouch-sensitive apparatus shown in FIG. 2A.

FIG. 4 illustrates a section that may be taken along any one of thedetection lines D of the touch-sensitive apparatus in FIG. 2A. Forsimplicity, only a portion of the apparatus at and around theoutcoupling structure is shown. The outcoupling structure is made up ofa sheet-like angular filter 20 which is applied with its front face tothe panel 1 at the periphery of the touch surface 4 to define anoutcoupling port for the light propagating in the panel 1. A detector 3is applied with its light-sensing surface 3A onto the rear face of theangular filter 20. In an alternative (not shown), a spacer of lighttransmissive material is disposed between the surface 3A and the filter20. As used herein, the detector 3 may be any device capable ofconverting light, at least within a portion of the wavelength range thatthe emitter operates, into an electrical signal, such as aphoto-detector, a CCD device, a CMOS device, etc. The angular filter 20is designed to only be transmissive to light within a given angularrange, i.e. for light that impinges on the filter 20 at certain anglesof incidence to the normal N (FIG. 5B) of the filter 20. FIG. 5A is aplot of the transmission of the filter 20 as a function of angle ofincidence. As seen, the filter 20 transmits light within a given angularrange between a lower limit θ_(min) and an upper limit θ_(max), whilereflecting light that impinges on the filter at other angles ofincidence. Any criterion may be used for defining the limits θ_(min),θ_(max), e.g. a percentage (e.g. 25, 50 or 75) of absolute or maximumtransmission. FIG. 5B illustrates the angular range in relation to asingle position on the front surface of the filter 20. As seen, theangular range is equal in all directions in the plane of the filter 20.Preferably, the angular range is essentially the same at all positionson the filter 20.

The filter 20 may be designed as a dielectric multilayer structure of atleast two different materials, similar to an interference filter. Itlies within the reach of the person skilled in optical design to selectappropriate materials and number of layers to achieve the desiredangular range for light at a wavelength generated by the emitters 2. Asan example, the article “Injecting Light of High-Power LEDs into ThinLight Guides”, by Cornelissen et al, published in Proc. SPIE 7652,International Optical Design Conference 2010, pp 7652121-7652126, 2010,discusses optical coupling in a backlight light guide. A dielectricmultilayer filter is deposited on the bottom of the light guide panel,and the top surface of a LED for injecting light into the light guide isoptically coupled to the filter by a silicone adhesive. The filter isoptimized to only transmit light emitted from the LED at angles largerthan the critical angle at the light guide-air interface. The purpose ofthe multilayer is thus to only transmit light that can propagate in thelight guide. The light emitted at smaller angles is reflected backtoward the rough LED surface where it is subsequently recycled byreflection and redistribution. Further elaboration on this incouplingapproach is given in the article “Dielectric multilayer angular filtersfor coupling LEDs to thin light guides”, by Mu et al, published in Proc.SPIE 8170, pp 8170011-81700110, 2011.

It should be understood that the filter 20 need not be designed todefine the given angular range for all wavelengths, but only for alimited wavelength range that includes the wavelength(s) of thepropagating light. In a preferred embodiment, the angular filter 20 isreflective to all angles outside the limited wavelength range, or atleast a range that is otherwise difficult to block with standard off theshelf daylight filters or dyes normally incorporated in detectorpackages. The filter may actually be constructed having a carrier layerthat is dyed or simply an ink jet painted thin layer in order tosimplify the multilevel design. Standard IR inks block effectively inthe visible area while transmitting over e.g. 750 nm. In this aspect,the use of a VCSEL array as emitter 2 provides an additional benefit,since it is very narrow light source. Typically, a VCSEL is a singlemode laser, and a VCSEL array emitter 2 may have a spectral width of <1nm FWHM (Full Width at Half Maximum). In addition, a VCSEL exhibits hightemperature stability, with very little drift of <0.1 nm/deg. It isthereby possible to have a very narrow band pass filter of less than 5nm. This way it is possible to block out virtually all ambient lightoutside the spectral range of the emitter, by means of an appropriateband pass filter, which may significantly increase the signal to noiseratio at the detector side. Such a band pass filter function may bearranged in the angular filter 20 by means of the structure and materialchoice of said dielectric layers in the filter 20. Alternatively, theband pass filter function may be provided as a separate element inaddition to the angular filter 20, e.g. if the angular filter is of amore mechanically vignetting nature as described below with reference toFIGS. 11 and 12.

It should also be understood that the filter 20 is designed to providethe angular range [θ_(min)−θ_(max)] for a specific installation, i.e.when mounted with its front face to the panel 1 and with its rear sideto the surface 3A (or a spacer). For example, the design may be adaptedto the refractive index of the panel 1 and the refractive index of thelight-sensing surface 3A (or the spacer).

In embodiments of the invention, the filter 20 is tailored to suppressthe amount of ambient light received at the light-sensing surface 3A inrelation to the amount of useful light, i.e. light that has propagatedon one or more detection lines from a respective incoupling structure.This effect may be achieved by adapting the angular range of the filter20 to the angles of incidence (AOI) of the propagating light on thefilter 20. Ambient light typically contains daylight and/or light fromartificial light sources. Such ambient light includes NIR light which,if it falls on the surface 3A, will interfere with the detection of thepropagating NIR light inside the panel 1. As exemplified by ray A1 shownin FIG. 4, ambient light that falls on the front surface 5 is refractedinto the panel 1 and would have impinged on the surface 3A were it notfor the angular filter 20. The angular filter 20 is designed to reflectthe ambient light A1 back towards the front surface 5. Ambient lightthat falls onto the touch surface 4 at other areas than over thedetector 3 will, irrespective of its angle to the touch surface 4, betransmitted through the panel 1, as exemplified by ray A2. The ambientlight will not be captured by TIR in the panel 1 since it cannot berefracted into the panel 1 at an angle larger than the critical angle(see below). As exemplified by ray P1, light that propagates in thepanel 1 by TIR and strikes the filter at an AOI within the angular rangeof the filter 20, is transmitted to the light-sensing surface 3A. FIG. 4also indicates a ray P2 that propagates in the panel 1 by TIR andstrikes the filter 20 at a smaller AOI, which is outside the angularrange of the filter 20. This ray P2 is reflected by the filter 20.However, with an emitter 2 in the form of a narrow beam VCSEL array, andcareful design of the filter 20, a substantial amount of light injectedfrom the emitter 2 may be used for detection by the detector 3. In otherwords, very few, if any, rays of light emanating from the emitter 2 willpropagate like ray P2 of FIG. 4. Preferably, the filter 20 is configuredto let through light to the detector 3 within an angular range which istailored to the beam divergence of the VCSEL array emitter 2. It shouldbe noted that a beam divergence of e.g. 19° (1/e²), such as offered inVCSEL arrays provided by Princeton Optronics Inc., translates to about12.5° (1/e²) in a panel with a refractive index of 1.5. It can therebybe understood that it is possible to make use of a very narrow angularfilter, virtually without sacrificing or losing any emitter light. Theuse of an angular filter 20 in combination with a VCSEL array emitter 2thus provides a simple technique for selecting the light that is passedto the detector 3 to be represented in the projection signals. Toachieve even better performance with a very narrow angular filter it maybe suitable to use e.g. wafer level optics in order to collimate thetheta range even further.

The present Applicant has realized that advantageous technical effectsmay be achieved by careful selection of the lower limit θ_(min) of thefilter 20. FIG. 6 illustrates, in section view, a portion of the panel1, where different characteristic angles of the panel 1, as well as thelimits θ_(min), θ_(max) of the filter 20, are mapped to a position onthe front surface 5. In this example, it is assumed that the panel 1 ismade of a single material and has the same refractive index n_(panel) atthe front and rear surfaces 5, 6. All angles are defined with respect tothe normal N of the front surface 5.

FIG. 6 indicates the critical angle θ_(c), which is given byθ_(c)=arcsin(1/n_(panel)) and is the minimum AOI of the light thatpropagates by TIR between the surfaces 5, 6. FIG. 6 also indicates acut-off angle θ_(w) for interaction between the propagating light andwater deposited on the front surface 5. Propagating light that strikesthe surface 5 at AOIs below θ_(w) will be partly coupled out of thepanel 1 and into water on the surface 5, whereas propagating light atAOIs above θ_(w) will be totally reflected at the interface between thesurface 5 and water. According to Snell's law: n_(panel)·sin(θ_(w))=n_(w)·sin(90°), which yields θ_(w)=arcsin(n_(w)/n_(panel)),where the refractive index of water, n_(w), is in the range 1.31-1.34,depending on temperature, wavelength, salt content, etc. In thefollowing examples, it is assumed that n_(w)=1.33. FIG. 6 also indicatesa cut-off angle θ_(f) for interaction between the propagating light andfinger fat deposited on the front surface 5. Propagating light thatstrikes the surface 5 at AOIs below θ_(f) will be partly coupled out ofthe panel 1 and into finger fat on the surface 5, whereas propagatinglight at AOIs above θ_(f) will be totally reflected at the interfacebetween the surface 5 and finger fat. According to Snell's law:n_(panel)·sin(θ_(f))=n_(f) sin(90°), which yieldsθ_(f)=arcsin(n_(f)/n_(panel)), where the refractive index of finger fat,n_(f), is in the range of 1.36-1.48, depending on temperature,wavelength, composition, etc. In the following examples, it is assumedthat n_(f)=1.45. In the example of a panel made of PMMA withn_(panel)=1.49, this approximately yields θ_(c)=42°, θ_(w)=63°, andθ_(f)=77°.

In one embodiment, the angular range is set toθ_(min)≈θ_(c)<θ_(max)<90°. This will ensure that all propagating lightreaches the light-sensing surface 3A while preventing a major part ofthe ambient light from striking the light-sensing surface 3A.

Further suppression of interferences, i.e. unwanted signal components atthe light-sensing surface 3A, may be achieved by setting θ_(min) toexceed θ_(c).

In one such embodiment, the angular range is set toθ_(min)=θ_(c)+Δθ<θ_(max)<90°, with Δθ equal to, or larger than, e.g. 5°,10° or 15°. This embodiment has been found to significantly reduce theinfluence of ambient light that is coupled into the panel 1 throughdeposits on the touch surface 4, such as water, saliva, fingerprints,smear, etc. (collectively denoted “contamination” herein). It will alsohelp suppressing other forwards dominated scattering factors (Mie type)e.g. from surface treatments such as antiglare and bulk scattering. Withreference to ray A2 in FIG. 4, it was mentioned that all ambient lightthat falls onto the touch surface 4 will pass through the panel 1.However, the present Applicant has found that this is only true for aperfectly clean touch surface 4. As indicated in FIG. 7, contamination30 on the touch surface 4 may allow ambient light at an angle ofincidence near or larger than θ_(c) to leak into the panel 1 and betrapped by TIR therein. The present Applicant has realized that thisambient light may have a sizeable influence on the measured signallevels at the detector 3. The Applicant has also found, surprisingly,that the trapped ambient light is largely concentrated to a limitedrange of AOIs at the filter 20, close to and above the critical angleθ_(c). Thus, the trapped ambient light can be prevented from reachingthe detector 3 by choosing θ_(min)=θ_(c)+Δθ with an angular span Δθequal to or larger than the limited range of AOIs. It is currentlybelieved that the concentration of the ambient light to the limitedrange of AOIs is caused by Mie scattering in the contamination 30, aswell as refraction by contamination droplets, causing the ambient lightto enter the panel with a slightly widened distribution, as indicated by32 in FIG. 7. The widening due to Mie scattering and refraction incontamination droplets may be in the range of ±5° to ±10°. The ambientlight typically falls onto the contamination 30 at many differentangles. Since all ambient light that enters the panel 1 at an anglebelow θ_(c) will pass through the panel 1 and since the intensity of theambient light typically decreases with increasing angle to the normal Nof the panel, the trapped ambient light will be concentrated at andslightly above θ_(c). It should also be noted that if the refractiveindex of the panel 1 is larger than the refractive index of thecontamination 30: n_(panel)>n_(cont), the AOIs for the ambient lightrefracted into the panel 1 via the contamination 30 cannot exceedarcsin(n_(cont)/n_(panel)).

The present Applicant has found that further advantageous and unexpectedeffects are achieved by designing the filter 20 with a given relationbetween the lower limit θ_(min) and the cut-off angle θ_(w) or θ_(f).

In one such embodiment, the angular range is set to θ_(min)≤θ≤θ_(max),where θ_(min)≥θ_(w) and θ_(max)<90°. Thus, the filter 20 is designed toonly transmit light with AOIs that are equal to or larger than thecut-off angle θ_(w) for water. This embodiment has the ability ofsignificantly reducing the influence on the resulting projection signalsfrom water-containing deposits on the touch surface 4. As noted above,the portion of the propagating light that strikes water at AOIs belowθ_(w) will be at least partially coupled out of the panel 1 and interactwith the water. Thus, the portion of the propagating light that reachesthe filter 20 at AOIs below θ_(w) has been significantly more attenuatedby water than the remainder of the propagating light. This embodimentalso has the ability of reducing the impact of differences in fingerinteraction between users and even between fingers of a single user.These differences may make it difficult to properly detect all touchingobjects on the touch surface, and it may require the signal processor 10to be configured with a large dynamic range for retrieving andprocessing the projection signals. A significant part of the differencesin finger interaction has been found to emanate from different moisturelevels on the fingers. The filter design of this embodiment willsuppress the influence of moisture in the projection signals and thusreduce the impact of differences in finger interaction.

In this embodiment, the propagating light that is transmitted by thefilter 20 has impinged on the touch surface 4 with AOIs at or aboveθ_(w). At these AOIs, the propagating light will still be coupled intothe outermost layer of the finger that form part of the epidermis, sincethis layer (stratum corneum) is known to have a refractive index ofabout 1.55 in the near infrared (NIR), e.g. according to measurementresults presented in “A survey of some fundamental aspects of theabsorption and reflection of light by tissue”, by R. J. Scheuplein,published in J. soc. cos. CHEM. 15, 111-122 (1964), and “The optics ofhuman skin”, by Anderson and Parrish, published in Journal ofInvestigative Dermatology 77, 1, 13-19 (1981). This means thatpropagating light is coupled into the finger for AOIs at least up to acutoff angle θ_(cs)=arcsin(1.55/n_(panel)) If n_(panel)<1.55, the cutoffangle θ_(cs) is not relevant, and all AOIs below 90° will interact withthe stratum corneum (and other outer layers of the finger). Ifn_(panel)>1.55, it is conceivable to set θ_(max)≤θ_(cs) for the filter20, should there be a need to suppress propagating light at AOIs aboveθ_(cs).

It should also be noted that this embodiment fully eliminates ambientlight that has been coupled into the panel via water on the touchsurface 4 and has propagated by TIR to the filter 20. As explainedabove, this ambient light has a maximum AOI ofarcsin(n_(cont)/n_(panel)) which is equal to θ_(w) with n_(cont)=n_(w).

In another embodiment, the angular range is set to θ_(min)≤θ≤θ_(max),where θ_(min)≥θ_(f) and θ_(max)<90°. This embodiment has the ability ofsignificantly reducing the influence on the resulting projection signalsfrom deposits containing finger fat, e.g. fingerprints, on the touchsurface 4. Fingerprints is typically a substantial part of thecontamination on the touch surface, and is a major concern whenprocessing the projection signals for detecting the touching objects. Itis thus a significant technical achievement to be able to suppress theinfluence of fingerprints, and it will reduce the requirements on thesignal processor 10 to track and compensate for contaminations. Thisembodiment also has the ability of further reducing the impact ofdifferences in finger interaction, since it suppresses the interactioncaused by fat on the fingers. Furthermore, this embodiment fullyeliminates ambient light that has been coupled into the panel via fingerfat on the touch surface 4 and has propagated by TIR to the filter 20.This ambient light has a maximum AOI of arcsin(n_(cont)/n_(panel)) whichis equal to θ_(f) with n_(cont)=n_(f).

Reverting to FIG. 4, the width W of the filter 20 (in the direction ofthe respective detection line) may be optimized with respect to therange of AOIs of the propagating light that should be transmitted ontothe light-sensing surface 3A. To ensure that all of this propagatinglight (i.e. the light with appropriate AOIs) strikes the filter at leastonce, the width W may be set to exceed the relevant minimum distancebetween bounces in the rear surface (“minimum bounce distance”). If thepanel is made of a single material, the minimum bounce distance is givenby 2·t·tan(θ_(min)), where t is the thickness of the panel 1. To achievea consistent detection of the propagating light within the limitsθ_(min), θ_(max), it may be desirable to set the width to exceed therelevant maximum distance between bounces in the rear surface (“maximumbounce distance”). If the panel is made of a single material, themaximum bounce distance is given by 2·t·tan(θ_(max)). In practice, thewidth W may be given by other design considerations, which may (but neednot) set the limit θ_(max) of the filter 20. Further, the skilled personis able to calculate the minimum bounce distance and the maximum bouncedistance for a panel consisting of more than one layer.

The foregoing design rules for the angular filter were given for a panel1 with a single index of refraction. However, corresponding design rulesare applicable for the angular filter 20 when applied to a panel 1 madeup of two or more layers with different index of refraction. FIG. 8illustrates a panel 1 formed by a top layer with index of refraction n₁and a bottom layer with index of refraction n₂. Light having an angle ofincidence θ₁ at the front surface 5 will impinge on the rear surface 6with an angle of incidence θ₂=arcsin(n₁/n₂ sin(θ₁)). This means that thelimits θ_(min), θ_(max) of the filter 20, if mounted on the rear surface6, should be set with respect to the critical angle θ_(c) at the frontsurface 5 as represented at the rear surface 6, or with respect to thecut-off angles θ_(w), θ_(f) at the front surface 5 as represented at therear surface 6. The skilled person realizes that n_(panel) in the aboveexpressions for θ_(c), θ_(w) and θ_(f) is the refractive index of thepanel 1 at the outcoupling region where the filter is mounted. In theexample of FIG. 8, n_(panel)=n₂.

A different situation may arise if the panel 1 is laminated to a display34 by means of a lamination layer 36, as shown in FIG. 9. If the display34 does not reflect light back to the panel 1, the lamination layer 36may be designed with a smaller refractive index n_(lam) than the panel.This will cause light that impinges on the interface between the paneland the lamination layer 36 at angles equal toθ_(c,l)=arcsin(n_(lam)/n_(panel)), or larger, to be totally reflected atthis interface. Light that impinges on the interface at smaller angles,e.g. as indicated by a dotted arrow in FIG. 9, will be transmitted viathe lamination layer 36 to the display 34. This means that the minimumAOI of the light that propagates by TIR between the surfaces 5, 6 andstrikes the filter 20, is θ_(c,l) rather than θ_(c). In such anembodiment the angular range of the filter 20 may be set according toθ_(min)≤θ≤θ_(max), where θ_(min)≥θ_(c,l) and θ_(max)<90°. Of course, theangular range may instead be designed with respect to θ_(w) or θ_(f),according to the embodiments described above.

It should be noted that a lamination layer 36 may generally beintroduced between the rear surface 6 of the panel 1 and any externaldevice when it is desirable to “optically isolate” the propagating lightin panel from the external device, whereby the propagating light isshifted to larger angles of incidence by virtue of θ_(c,l)>θ_(c). Basedon the foregoing discussion, it is understood that it may be desirableto select the material of the lamination layer 36 such thatθ_(c,l)≥θ_(w) or θ_(c,l)≥θ_(f), so as to reduce the attenuation causedby contamination on the touch surface 4. In another variant, it may bedesirable to select the material of the lamination layer 36 such thatθ_(c,l)≥θ_(c)+Δθ, where Δθ is selected to reduce the influence ofambient light that enters the panel via contamination on the touchsurface 4, as discussed above with reference to FIG. 7. For efficientutilization of light, the light emitters 2 may be coupled to the panel 1such that the injected light impinges on the lamination layer 36 at anAOI that exceeds θ_(c,l), to avoid that useful light is leaked into thelamination layer 36. The concept of using a lamination layer 36 forreducing the impact of ambient light or contamination may, but need not,be used in combination with an angular filter in the outcouplingstructure and/or an angular filter in the incoupling structure (seebelow).

In the foregoing, it has been assumed that the cutoff angles θ_(w) andθ_(f) are given by the TIR angle at the interface between the panel andwater and finger fat, respectively. However, it shall be appreciatedthat the TIR angles correspond to 100% reflection at the interface, andthat the reflectivity at the interface does not exhibit a step change atthe TIR angle but is a continuous, but steep, function within increasingAOI until the TIR angle. This is illustrated in FIG. 15, which is agraph of the reflectivity R for unpolarised light at a panel-to-waterinterface as a function of AOI, θ, given by the equations:

$R_{p} = {\frac{{n_{panel}*\cos\;\theta} - {n_{w}*\sqrt{1 - ( {\frac{n_{panel}}{n_{w}}\sin\;\theta} )^{2}}}}{{n_{panel}*\cos\;\theta} + {n_{w}*\sqrt{1 - ( {\frac{n_{panel}}{n_{w}}\sin\;\theta} )^{2}}}}}^{2}$$R_{s} = {\frac{{n_{panel}*\sqrt{1 - ( {\frac{n_{panel}}{n_{w}}\sin\;\theta} )^{2}}} - {n_{w}*\cos\;\theta}}{{n_{panel}*\sqrt{1 - ( {\frac{n_{panel}}{n_{w}}\sin\;\theta} )^{2}}} + {n_{w}*\cos\;\theta}}}^{2}$$R = \frac{R_{s} + R_{p}}{2}$

with n_(panel)=1.51 and n_(w)=1.33.

As understood from FIG. 15, it is possible to define the cutoff anglesθ_(w) and θ_(f) at a given fraction of 100% reflection, e.g. 0.25 or0.50, while still achieving an essentially complete suppression of theinfluence of water and finger fat, respectively. In practice, this meansthat it is possible to adjust the cutoff angles slightly from the TIRangles towards smaller AOIs: θ_(w)=arcsin(n_(w)/n_(panel))−δθ andθ_(f)=arcsin(n_(f)/n_(panel))−δθ, where δθ is typically less than 2°. Asused herein, any reference to θ_(w) and θ_(f) is intended to inherentlyinclude this minor shift δθ.

Generally, it may be desirable to limit the size of the individualdetectors 3, and specifically the extent of the light-sensing surface3A. For example, the cost of light detectors may increase with size.Also, a larger detector typically has a larger capacitance, which maylead to slower response (longer rise and fall times) of the detector. Itis realized that it may be difficult to reduce the extent W of thedetector 3 in the embodiment of FIG. 4, without sacrificing the abilityto consistently detect the propagating light.

FIG. 10A illustrates an alternative embodiment that at least partlyovercomes this problem. Like in FIG. 4, the angular filter 20 is appliedto the rear surface 6, but the detector 3 is arranged with itslight-sensing surface 3A essentially perpendicular to the main extent ofthe panel 1. As used herein, “essentially perpendicular” is intended toinclude deviations of about ±20° or less from perpendicular. A spacer 22is disposed between the angular filter 20 and the light-sensing surface3A. The spacer 22 may be made of any suitable light transmissivematerial, such a plastic material or glass, or a silicone compound, aglue, a gel, etc. In the example of FIG. 10A, the spacer 22 is made ofthe same material as the panel 1, i.e. n_(spacer)=n_(panel). Byarranging the surface 3A vertically, the extent of the surface 3A may bereduced significantly compared to the embodiment in FIG. 4. When theextent of the angular filter is W, the minimum vertical extent of thesurface 3A may be given by W/tan(θ_(min)) to ensure that all of thetransmitted light from the filter 20 is received at the surface 3A. Theextent of the surface 3A may be reduced further by selecting thematerial of the spacer 22 such that n_(spacer)<n_(panel), causing thetransmitted light to be refracted away from the normal and resulting ina smaller projected height at the location of the surface 3A.

Even if the outcoupling structure in FIG. 10A allows for a smallerlight-sensing surface 3A, the extent of the surface 3A is stilldependent on the angular range of the filter 20. Further, the verticalextent of the surface 3A has a direct impact on the thickness of theapparatus 100. At present, the embodiment in FIG. 10A is believed to beuseful only when θ_(min) is larger than about 45°, to avoid that theheight of the outcoupling structure becomes excessive.

This problem is at least partly overcome by the embodiment illustratedin FIGS. 10B-10C. As shown in section in FIG. 10B, the angular filter 20is applied to the rear surface 6, and a light recycler 40 is arrangedbeneath the filter 20 in surrounding relationship to the light-sensingsurface 3A. The recycler 40 is designed to internally reflect the lightthat is transmitted by the filter 20 and to modify the angulardistribution of the transmitted light. The recycler 40 defines areflective enclosure around the light-sensing surface 3A. The enclosureis filled by a spacer material 22, e.g. any of the spacer materialsdiscussed in relation to FIG. 10A. The recycler 40 comprises areflective bottom surface 42A with an opening for the detector 3(illustrated as mounted on a PCB 45) and reflective sidewalls 42B thatextend from the bottom surface 42A to the filter 20. In the illustratedembodiment, the bottom surface 42A is diffusively reflective toimpinging light, whereas the sidewalls 42B are specularly reflective toimpinging light. As used herein, “specular reflection” is given itsordinary meaning, which refers to the mirror-like reflection of lightfrom a surface, in which light from a single incoming direction (a ray)is reflected into a single outgoing direction. Specular reflection isdescribed by the law of reflection, which states that the direction ofincoming light (the incident ray), and the direction of outgoing lightreflected (the reflected ray) make the same angle with respect to thesurface normal, and that the incident, normal, and reflected directionsare coplanar. As used herein, “diffuse reflection” is given its ordinarymeaning, which refers to reflection of light from a surface such that anincident ray is reflected at many angles rather than at just one angleas in specular reflection. The diffuse reflection is also known as“scattering”. The skilled person appreciates that manysurfaces/elements/materials exhibit a combination of specular anddiffuse reflection. As used herein, a surface is considered “diffusivelyreflective” when at least 20% of the reflected light is diffuse. Therelation between diffuse and specular reflection is a measurableproperty of all surfaces/elements/materials.

To exemplify the function of the outcoupling structure, FIG. 10Billustrates a single ray P1 that is transmitted and refracted into therecycler 40 by the filter 20. The ray strikes the bottom wall 42A and isdiffusively reflected, as indicated by the encircled rays 46. Thediffusively reflected light spreads over a large solid angle in therecycler 40, and some of this light is specularly reflected by thefilter 20 onto the light-sensing surface 3A. Although not shown, otherparts of the diffusively reflected light is likely to undergo furtherreflections in the recycler 40, against the sidewalls 42B, the angularfilter 20 and the bottom wall 42A, and eventually impinge on the surface3A. It should be noted that the angular filter 20 will also transmitlight from the recycler 40 back into the panel 1, specifically lightthat has an angle of incidence within a given angular range which may,but need not, be identical to the angular range [θ_(min)−θ_(max)]. Ifthese angular ranges are identical, or at least substantiallyoverlapping, it is necessary to redistribute the light by diffusereflection inside the recycler 40 to prevent that the light that entersthe recycler 40 from the panel 1 escapes back into the panel 1 via thefilter 20.

In the embodiment of FIG. 10B, the use of specular side walls 42Bensures that all of the light that enters the recycler 40 via the filter20 is re-directed by specular reflection(s) towards the bottom of therecycler 40, where it is either redistributed by diffuse reflection inthe bottom wall 42A or directly received by the light-sensing surface3A. The diffusively reflected light is typically, but not necessarily,emitted with a main direction that is perpendicular to the bottom wall42A, as shown in FIG. 10B. The bottom wall 42A may e.g. be anear-Lambertian diffuser. The use of a planar bottom wall 42A, as inFIG. 10B, ensures that most of the diffusively reflected light hits thefilter 20 at AOIs outside the angular range, such that main portion ofthe diffusively reflected light is reflected by the filter 20 back intothe recycler 40.

In an alternative, both the bottom wall 42A and the side walls 42B arediffusively reflective. In another alternative, the side walls 42B arediffusively reflective, while the bottom surface 42A is specularlyreflective. In all embodiments, it is possible that only a part of thebottom wall 42A and/or the side walls 42B is diffusively reflective.Generally, it may be advantageous to provide diffusive scattering onsurfaces that are arranged such that a significant portion of the lightimpinging on these surfaces would otherwise be specularly reflected ontothe filter 20 within the angular range.

In yet another alternative, the bottom wall 42A is not specularly ordiffusively reflective, but provided with a micro-structure, which isconfigured to reflect and re-direct impinging light onto the lightsensing-surface 3A, by specular reflection against the filter 20 andpossibly by reflection against the specular side walls 42B. Themicro-structure thus forms a mirror with an optical power that istailored to the incoming light, i.e. the light that is transmitted bythe filter 20 and hits the micro-structure on the bottom wall 42A,either directly or by reflection(s) in the side walls 42B. The use ofspecularly reflective side walls 42B may facilitate the design of themicro-structure, but it is possible to use diffusively reflective sidewalls 42B, or a combination thereof. The micro-structure may beimplemented as a sheet-like Fresnel mirror.

Compared to the embodiments in FIG. 4 and FIG. 10A, there is no directrelation between the extent of the angular filter 20 and the requiredsize of the light-sensing surface 3A, since the recycler 40 is designedto retain a portion of the transmitted light by internal reflectionsuntil it impinges on the surface 3A. Furthermore, there is large freedomof placing the detector 3 in relation to the recycler 40, and it caneven be accommodated in a side wall 42B instead of the bottom wall 42A.It is realized that the outcoupling structure may be optimized withrespect to manufacturing requirements, without any major loss inoutcoupling efficiency. Also, assembly tolerances may be relaxed.

As shown in the plan view of FIG. 10C, the combination of angular filter20 and recycler 40 will collect light from all directions in the planeof the panel 1, provided that the incoming light impinges on the angularfilter 20 with AOIs within the angular range [θ_(min)−θ_(max)]. Thus,the outcoupling structure in FIG. 10C can accept light from manydifferent directions in the plane of the panel 1 and define the endpoint of detection lines from different emitters (cf. FIG. 2A). In theexample of FIG. 10C, the recycler 40 is circular in plan view, but othershapes are possible, e.g. elliptical or polygonal. As described inrelation to FIG. 4, it may be desirable that the extent W of therecycler along each of the detection lines is equal to at least theminimum bounce distance or at least the maximum bounce distance.

In all embodiments, the specularly reflective wall(s) of the recycler40, if present, may be implemented by an external coating, layer or filmwhich is applied to the spacer material 22, e.g. a metal such asaluminum, copper or silver, or a multilayer structure, as is well-knownto the skilled person.

In all embodiments, the diffusively reflective wall(s) of the recycler40, if present, may be implemented by an external coating, layer or filmof diffusively reflective material which is applied to the spacermaterial 22. In one implementation, the diffusively reflective materialis a matte white paint or ink. In order to achieve a high diffusereflectivity, it may be preferable for the paint/ink to contain pigmentswith high refractive index. One such pigment is TiO₂, which has arefractive index n=2.5-2.7. It may also be desirable, e.g. to reduceFresnel losses, for the refractive index of the paint binder (vehicle)to match the refractive index of the spacer 22. For example, dependingon refractive index, a range of vehicles are available such as oxidizingsoya alkyds, tung oil, acrylic resin, vinyl resin and polyvinyl acetateresin. The properties of the paint may be further improved by use ofe.g. EVOQUE™ Pre-Composite Polymer Technology provided by the DowChemical Company. There are many other diffusively reflective coatingmaterials that are commercially available, e.g. the fluoropolymerSpectralon, polyurethane enamel, barium-sulphate-based paints orsolutions, granular PTFE, microporous polyester, Makrofol® polycarbonatefilms, GORE® Diffuse Reflector Product, etc. Also, white paper may beused. Alternatively, the diffusively reflective material may be aso-called engineered diffuser. Examples of engineered diffusers includeholographic diffusers, such as so-called LSD films provided by thecompany Luminit LLC.

According to other alternatives, the diffusively reflective wall(s) ofthe recycler 40 may be implemented as a micro-structure in the spacermaterial 22 with an overlying coating of specularly reflective material.The micro-structure may e.g. be provided in the spacer material 22 byetching, embossing, molding, abrasive blasting, etc. Alternatively, themicro-structure may be attached as a film or sheet onto the spacermaterial 22. The above-described mirror with an optical power tailoredto incoming light may also be provided as a micro-structure in or on thespacer material 22.

Below follows a description on variants of the outcoupling structure inFIG. 4. Unless explicitly excluded, the bottom wall 42A and the sidewalls 42B may be configured according to any of the embodiments,alternatives and variants described in relation to FIGS. 10B-10C.

FIG. 10D illustrates an outcoupling structure that may improve theability of the outcoupling structure to achieve a consistent detectionof the propagating light within the limits θ_(min), θ_(max), when thewidth W of the angular filter 20 and the light-sensing surface 3A issmaller than the maximum bounce distance or even smaller than theminimum bounce distance. This is achieved by providing the edge surface,which connects the front and rear surfaces 5, 6, with a specularlyreflective coating 48 at the location of angular filter 20. As shown,the coating 48 will operate to reflect, back into the panel 1, thepropagating light that passes above the angular filter 20, includingpropagating light that strikes the filter 20 at AOIs outside the angularrange θ_(min)−θ_(max) and propagating light that does not strike theangular filter 20. It is realized that the coating 48 will reflect theincoming light without changing its AOIs. Thus, a portion of thereflected light will have AOIs that fall within the angular range of thefilter 20. Such reflected light will be transmitted by the filter 20 tothe extent that it strikes the filter 20 on its way back towards thetouch surface 4. Thereby, the reflective coating 48 operates to increasethe outcoupling efficiency.

It is not strictly necessary to arrange the angular filter 20 directlyadjacent to the edge surface for the coating 48 to perform its functionof increasing outcoupling efficiency. However, such a placement may bepreferred to ensure that light on different detection lines can bereflected by the coating 48 onto the angular filter 20.

The reflective coating 48 may also be implemented to increaseoutcoupling efficiency in the embodiment of FIGS. 10B-10C.

FIG. 10E illustrates a variant of the outcoupling structure in FIG. 10B.The recycler 40 is configured as a hollow case, i.e. without spacermaterial. This is achieved by providing the angular filter 20 with adiffusively transmitting element 52 on its exit surface, such that thelight that enters the recycler is diffusive. Since light is angularlyredistributed on entry, the walls 42A, 42B may, but need not, bespecularly reflective.

FIG. 10F illustrates a further variant of the outcoupling structure inFIG. 10B. Here, the detector 3 is side-looking when mounted on a PCB 45.The detector 3 is inserted into the spacing material 22 such that thelight-sensing surface 3A projects beyond the bottom surface 42A of therecycler 40 and is perpendicular to the extent of the panel 1 and theangular filter 20. By arranging a side-looking detector 3 inside therecycler 40, it is possible to provide an increased light-sensingsurface 3A for a given opening in bottom surface 42A, especially if thelight-sensing surface 3A extends around the perimeter of the detector 3.The increased light-sensing surface 3A may result in an increasedoutcoupling efficiency. In a variant, the top surface of the detector 3may be provided with a diffusively reflective structure to enhance theangular redistribution of light inside the recycler 40.

FIG. 10G illustrates a further variant of the outcoupling structure inFIG. 10B, in which the recycler 40 is configured as a funnel tailored todirect the light transmitted by the filter 20 onto the light-sensingsurface 3A. The funnel is defined by the walls 42A, 42B, which arespecularly reflective. It should be noted that the recycler 40 may haveother shapes than shown in FIG. 10G. For example, the bottom wall 42Amay be parabolic. Other suitable shapes may be obtained by opticalsystem optimization, such as by ray tracing. Furthermore, the walls 42A,42B may be merged into a common funnel structure, which may be parabolicor have any optimized shape. The recycler 40, and optionally the filter20, may be integrated with the detector 3 into a package, which isattached to the rear surface 5 during manufacture of the apparatus.

FIG. 10H illustrates a variant of the outcoupling structure in FIG. 4.In FIG. 10H, the apparatus 100 further comprises a visibility filter 50,which is arranged to hide the angular filter 20, the detector 3 and theinternal structure of the apparatus 100 from view through the frontsurface 5. The visibility filter 50 is non-transmissive (opaque) tovisible light and transmissive to NIR light, and preferably onlytransmissive to NIR light in the wavelength region of the propagatinglight. The visibility filter 50 may be implemented as a coating or film,in one or more layers. In FIG. 10H, the visibility filter 50 extendsfrom the inner edge of the angular filter 20 to the edge of the panel 1,although the visibility filter 50 may extend further beyond the angularfilter 20 towards the center of the panel. In FIG. 10H, the visibilityfilter 50 is arranged beneath the panel 1, intermediate the rear surface6 and the angular filter 20. This enables the front surface 5 to beperfectly flat and free of projecting elements. In a variation, notshown, the visibility filter 50 is applied to the front surface 5. It isto be understood that the visibility filter 50 may be implemented inconjunction with any outcoupling structure described herein.

There are other ways of integrating the angular filter in theoutcoupling structure than by arranging the above-described multilayerstructure in front of the detector 3. For example, the angular filtermay be formed by a structure that geometrically and mechanically limitsthe light rays that can reach the detector, as exemplified below withreference to FIGS. 11-12.

FIG. 11A is a side view of a first alternative outcoupling structure asattached to the panel 1, and FIG. 11B is a top plan view of theoutcoupling structure. The outcoupling structure forms an angular filter20 by defining an angularly limited propagation path from an outcouplingport 60 to the light-sensitive surface 3A. The angular filter 20 isdefined by non-transmissive layers 61-63 on a body of light transmissivematerial, e.g. any of the spacer materials discussed in relation to FIG.10A. Layers 61 and 62 are arranged on top of the body, to define theoutcoupling port 60 at the interface between the body and the panel 1.Layer 61 is light-absorbing and formed as an annulus segment. Layer 61only needs to be absorbing to light inside the body. Layer 62 isspecularly reflective and formed as a semi-circle, which may extend tothe inner radius of the annulus segment, as shown, or to the outerradius of the annulus segment (i.e. between layer 61 and the panel 1).The top of the body is attached to the rear surface 5 of the panel 1,and the light detector 3 is attached to a short-side of the body suchthat the surface 3A is shielded beneath the layers 61, 62. Layer 63 isspecularly reflective and attached to bottom side of the body.

FIG. 11A illustrates one propagating ray P1 that passes the outcouplingport 60 at an AOI within the angular range of the filter 20 and isreflected by layer 63 onto surface 3A. FIG. 11A also illustrates apropagating ray P2 that propagates in the panel 1 by TIR and passes theoutcoupling port 60 and is reflected by layer 63 onto layer 61 whichabsorbs the ray. As seen, the structure of layers 61-63 define theangular range of the filter 20. The width of the layer 61 along therespective detection line defines θ_(min), for a given vertical heightof the body. To achieve similar θ_(min) for all detection lines in theplane of the panel, layer 61 is shaped as an annulus segment, althoughmore complicated shapes are conceivable to achieve a correspondingeffect.

FIG. 12A is a side view of a second alternative outcoupling structure asattached to the panel 1, and FIG. 12B is a top plan view of theoutcoupling structure. Like the first variant in FIG. 11, theoutcoupling structure forms an angular filter 20 by layers 71-73attached to a body of light transmissive material which is attached tothe rear surface 5 of the panel 1. Layer 71 is arranged on top of thebody, to define an outcoupling port 70 at the interface between the bodyand the panel 1. Layer 71 is specularly reflective and, in this example,formed as a semi-circle. Layer 72 is light-transmissive and attached tothe bottom surface of the body, and has a refractive index that causeslight that impinges on the outcoupling port 70 at AOIs above θ_(min) tobe reflected by TIR at the interface between the body and layer 72,whereas light at smaller AOIs is transmitted into layer 72 and isabsorbed by layer 73 of light-absorbing material. It is realized thatthe refractive index of layer 72 is given by n₇₂=n_(panel)·sin(θ_(min))The function of the outcoupling structure exemplified by one ray P1 thatpasses the filter 20 and one ray P2 that is coupled into layer 72.

In both of the first and second variants, it is conceivable that therefractive index n_(spacer) of the body is selected to yield a desiredθ_(max) by TIR in the interface between the body and the panel:n_(spacer)=n_(panel)·sin(θ_(max)).

Embodiments of structures for incoupling of light will now be explainedin detail with reference to FIG. 13. Generally, these embodiments arepresented in the context of the touch-sensitive apparatus shown in FIG.2A.

FIG. 13A illustrates a section that may be taken along any one of thedetection lines D of the touch-sensitive apparatus in FIG. 2A. For thesake of simplicity, only a portion of the apparatus at and around theincoupling structure is shown. The embodiment of FIG. 13A is a rear sidecoupled solution, in which the incoupling structure comprises a prism 25configured to direct light from the VCSEL array emitter 2 into the panel1. More specifically, the prism 25 is preferably configured so as tocouple light from the emitter 2 into the panel 1 along a general beamdirection having an advantageous angle θ of incidence to the front 5 andrear surfaces 6. Obviously, the angle θ shall be larger than thecritical angle θ_(c). Preferably, the beam angle θ is such thatsubstantially all light from emitter 2 falls within the angular rangeθ_(min)−θ_(max) of the angular filter 20. Recall that in a panel 1 witha refractive index of 1.5 the beam divergence of a VCSEL array emitter 2may be in the region of 12°. The prism 25 may be made from the samematerial as the panel 1, or any other material which is transmissive tothe wavelength of the emitter 2. Dependent on the specificconfiguration, the prism 25 may also have a reflective surface 27,rather than having a simply refractive function. For the purpose ofoptimizing the optical coupling, an index-matching component 29 mayfurther be disposed to link the emitter 2 to the prism 25, such as e.g.a layer of silicone.

FIG. 13B illustrates a bottom plan view of an embodiment of theincoupling structure of FIG. 13A. While it is beneficial that theemitter 2 has a narrow beam divergence vertically, i.e. in the θdirection, it is in some embodiments nevertheless preferable that thedivergence φ in the horizontal plane is comparatively wide. The reasonis that light from one emitter 2 then can be detected by severaldetectors 3 dispersed along the periphery of the touch surface 4, asseen in FIG. 2A. One way of obtaining this is to sweep the beam from theemitter 2 over the angular range of interest in the plane of the panel1. Another way is to optically modify the beam shape from the emitter 2,so as to be more fan-shaped. FIG. 13B schematically illustrates one wayof obtaining this effect. In this embodiment, the surface 27 of theprism 25 in which the beam from the VCSEL array emitter 2 is reflected,has a microstructure with repeated curved portions, each portion forminga convex mirror from the inside of the prism 25. This way, light fromthe emitter 2 may be dispersed in the horizontal plane of the panel, asschematically indicated by the dashed arrows. In the drawing of FIG. 13Bthere is no index-matching component 29 illustrated, but such acomponent may of course still be employed.

FIG. 13C illustrates an alternative to the embodiments of FIGS. 13A and13B. This embodiment makes use of an edge-coupled design, i.e. wherelight from the emitters 2 is coupled in through a side edge 28 of thepanel 1 rather than through the front 5 or rear 6 surface. In theexemplary embodiment of FIG. 13C, a beam direction from the VCSEL arrayemitter 2 with a suitable angle of incidence to the front 5 and rear 6surfaces is obtained by means of a wedged shape of the side edge 28, atleast at the incoupling site. The VCSEL array emitter 2 may be attacheddirectly to the side edge 28 by means of an index-matching adhesive suchas silicone 29. An alternative solution (not shown) may be to couplelight from the emitters 2 to the side edge 28 by means of an opticalfiber. The embodiment of FIG. 3C is only presented in a side sectionalview, but it should be understood that also in this embodiment theincoupling structure at 28 may be configured to spread the emitter lightin a fan shape in the panel.

The incoupling structures of FIGS. 13 A-C are intended to illustrate afew different ways of coupling light into the panel 1 at a suitableangel, but the shown embodiments are not intended to be seen asexclusive examples. Also, it should also be noted that theabove-described visibility filter (cf. 50 in FIG. 10H) may beimplemented in conjunction with any incoupling structure describedherein, e.g. intermediate the panel 1 and the emitter 2, even though thevisibility filter has been left out in FIGS. 13A-C.

FIG. 14 is a bottom plan view of a part of a touch-sensitive apparatus,to illustrate a sequence of emitters 2 and/or detectors 3 that areoptically coupled to a panel 1 near one of the panel edges, via avisibility filter 50 which is applied to the rear surface 6 to extend asa coherent, elongate strip along the panel edge. Each emitter 2 anddetector 3 is coupled to the panel 1 by a coupling structure. For theemitters 2 the coupling structure may comprise a prism 25, and for thedetectors 3 this coupling structure may comprise an angular filter 20and a recycler 40. The combination of angular filters 20 and recyclers40 are arranged as discrete units along the panel edge. It shall benoted that the neither the shape nor the specific location of therespective emitters 2 and detectors 3, and their coupling structures,necessarily reflect an optimum configuration. On the contrary, FIG. 14is merely intended to provide a schematic representation to indicate thedistribution of the emitters 2 and detectors 3 over a common visibilityfilter 50.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andthe scope of the appended claims.

The foregoing description indicates that certain selections of the lowerlimit θ_(min) (e.g. with respect to the cutoff angles θ_(c,l), θ_(w),θ_(f)) results in significant performance improvement. However, it ispossible that performance improves gradually as the lower limit θ_(min)is increased from the critical angle θ_(c), e.g. in steps 1°, at leastin certain installations. Testing of a certain installation may thusindicate that an acceptable performance improvement is attained atanother selected θ_(min), e.g. any angle between θ_(c) and θ_(w),between θ_(w) and θ_(f), and between θ_(f) and θ_(sc). The same appliesto the selection of refractive index of the lamination layer forsuppressing the influence of contamination via the cutoff angle θ_(c,l).

The invention claimed is:
 1. A touch-sensitive apparatus, comprising: alight transmissive panel comprising a front surface and an opposite,rear surface; a plurality of light emitters configured to opticallyconnect to the panel and further configured to generate light thatpropagates by total internal reflection inside the panel across atouch-sensitive region on the panel; and a plurality of light detectorsconfigured to optically connect to the panel and further configured todefine a grid of propagation paths across the touch-sensitive regionbetween pairs of light emitters and light detectors; wherein each ofsaid light emitters comprises a vertical-cavity surface-emitting laser(“VCSEL”) array, each said VCSEL array including a plurality of VCSELsconfigured to be driven in parallel to collectively form one lightemitter; and wherein a bandpass filter, tailored to an operatingwavelength for said VCSELs, is arranged at said detectors.
 2. Thetouch-sensitive apparatus of claim 1, wherein each light detector isoptically connected to the light transmissive panel via an angularfilter which is applied to an outcoupling region of the lighttransmissive panel and is configured to transmit the propagating lightonly within a confined range of angles with respect to the normal of theoutcoupling region; and wherein each light emitter is opticallyconnected to an incoupling region of the light transmissive panel and isconfigured to transmit a beam of light for total internal reflection soas to reach the outcoupling region at least partially within saidconfined range of angles.
 3. The touch-sensitive apparatus of claim 2,wherein the confined range extends from a lower angle limit θ_(min) toan upper angle limit θ_(max), wherein the lower angle limit θ_(min) isequal to or larger than a critical angle θ_(c), which is given byθ_(c)=arcsin(1/n_(panel)), with n_(panel) being the refractive index ofthe light transmissive panel at the outcoupling region.
 4. Thetouch-sensitive apparatus of claim 3 , wherein the lower angle limitθ_(min) exceeds the critical angle by an angle Δθ, which is at least 5°,10° or 15°.
 5. The touch-sensitive apparatus of claim 3, wherein thelower angle limit θ_(min) is equal to or larger than a first cut-offangle θ_(w)=arcsin(n_(w)/n_(panel)), with n_(w) being the refractiveindex of water, n_(panel)>n_(w).
 6. The touch-sensitive apparatus ofclaim 3, wherein the lower angle limit θ_(min) is equal to or largerthan a second cut-off angle θ_(f)=arcsin(n_(f)/n_(panel)), with n_(f)being the refractive index of finger fat, n_(panel)>n_(f).
 7. Thetouch-sensitive apparatus of claim 3, wherein the light transmissivepanel is mounted onto a front surface of a display device by alamination layer of light transmissive material which is arranged incontact with the rear surface of the light transmissive panel and thefront surface of the display device, and wherein the lower angle limitθ_(min) is equal to or larger than a laminate cut-off angleθ_(c,l)=arcsin(n_(lam)/n_(panel)), with n_(lam) being the refractiveindex of lamination layer, n_(lam)<n_(panel).
 8. The touch-sensitiveapparatus of claim 2, wherein said outcoupling region is arranged on atleast one of the front and rear surfaces.
 9. The touch-sensitiveapparatus of claim 2, wherein said incoupling region is arranged on atleast one of the front and rear surfaces.
 10. The touch-sensitiveapparatus of claim 2, wherein said incoupling region is arranged on aside edge of the panel, connecting the front and rear surfaces.
 11. Thetouch-sensitive apparatus of claim 2, wherein the angular filter isconfigured as a dielectric multilayer structure.
 12. The touch-sensitiveapparatus of claim 1, wherein said bandpass filter has a bandwidth ofless than 5nm.
 13. A touch-sensitive apparatus, comprising: a lighttransmissive panel comprising a front surface and an opposite, rearsurface; a plurality of light emitters configured to optically connectto the panel and further configured to generate light that propagates bytotal internal reflection inside the panel across a touch-sensitiveregion on the panel; and a plurality of light detectors configured tooptically connect to the panel and further configured to define a gridof propagation paths across the touch-sensitive region between pairs oflight emitters and light detectors; wherein each of said light emitterscomprises a vertical-cavity surface-emitting laser (“VCSEL”) array, eachsaid VCSEL array including a plurality of VCSELs configured to be drivenin parallel to collectively form one light emitter; wherein an angularfilter is configured to be applied to an outcoupling region of the lighttransmissive panel and is further configured to transmit the propagatinglight only within a confined range of angles with respect to the normalof the outcoupling region; and wherein the confined range extends from alower angle limit θ_(min) to an upper angle limit θ_(max), wherein thelower angle limit θ_(min) is equal to or larger than a critical angleθ_(c), which is given by θ_(c)=arcsin( 1/n_(panel)), with n_(panel)being the refractive index of the light transmissive panel at theoutcoupling region.
 14. A touch-sensitive apparatus, comprising: a lighttransmissive panel comprising a front surface and an opposite, rearsurface; a plurality of light emitters configured to optically connectto the panel and further configured to generate light that propagates bytotal internal reflection inside the panel across a touch-sensitiveregion on the panel; and a plurality of light detectors configured tooptically connect to the panel and further configured to define a gridof propagation paths across the touch-sensitive region between pairs oflight emitters and light detectors; wherein each of said light emitterscomprises a vertical-cavity surface-emitting laser (“VCSEL”) array, eachsaid VCSEL array including a plurality of VCSELs configured to be drivenin parallel to collectively form one light emitter; wherein an angularfilter is configured to be applied to an outcoupling region of the lighttransmissive panel and is further configured to transmit the propagatinglight only within a confined range of angles with respect to the normalof the outcoupling region; and wherein the angular filter is configuredas a dielectric multilayer structure.